Systems are known in the electrical discharge machining art in which electrical parameters are responded to when these are of an abnormal level to provide interruption of the power supply particularly in response to failure of output electronic switches in the power supply. The present invention is related to an embodiment of an EDM power supply in which the output switches are transistors although it is equally applicable to any precision type EDM power supply in which electronic switches are used to provide machining power pulses to the machining gap. By "electronic switch" we mean any electronic control device having a plurality of electrodes including at least two principal or power conducting electrodes acting to control current flow in the power circuit, the conductivity of the power circuit being controlled by a control electrode within the switch so the conductivity of the power circuit is statically or electrically without the movement of any mechanical elements within the switch. Included within the definition by way of illustration but not limitation are electronic tubes, transistors, semi-conductor controlled rectifiers and the like.
Various prior art systems have been developed to provide for interruption of machining current responsive to output transistor failure, which failure normally occurs in a shorted condition between the principal or power conducting electrodes of the output transistor. One example of such a protection circuit is disclosed and claimed in Robert S. Webb U.S. Pat. No. 3,257,580 issued on June 21, 1966 for "Fault Detection and Cut-Off Circuit for Electrical Discharge Machining Apparatus", which patent is of common ownership with the present application. The system described in that patent relates to a fault detection sensing which occurs during the machining power pulse off-time across the principal electrodes of the output switch and is thus operable during machining pulse off-time. A further example of the transistor failure cut-off detection circuit is also shown in Kurt H. Sennowitz U.S. Pat. No. 3,524,037 issued on Aug. 11, 1970 for "Fail Transistor Protective Circuit For Electrical Discharge Machining Apparatus", in which drive voltage level is sensed and a signal stage with cut-off switch is used to initiate relay interruption of the power supply.
An additional system for protecting against output transistor failure is shown and described in Oliver A. Bell, Jr. U.S. Pat. No. 3,751,622 issued on Aug. 7, 1973 for "Protection Circuit For Electrical Discharge Machining Power Supply Apparatus". In that patent which likewise is of common ownership herewith, the protection circuit senses drive voltage for the electronic output switch and uses a field effect transistor to initiate interruption of the power supply when output transistor failure occurs.
A still further system for protecting an EDM power supply system against damage resultant from output transistor failure is shown in Oliver A. Bell, Jr. U.S. Pat. No. 3,825,713 issued on July 23, 1974 for "Function Control Circuit for EDM Power Supply". In that patent, magnitude of current flow through a resistor in series with the output transistor principal electrode is sensed by means of a light emitting device and in that way a control signal for interrupting the power supply is generated. Bell U.S. Pat. No. 3,825,713 is likewise of common ownership with the present application.